1. Field of the Invention
The present invention relates to a semiconductor element and method of manufacturing the same. More particularly, it relates to a semiconductor element having a superjunction structure including p-type pillars and n-type pillars laterally and alternately buried in a drift layer and method of manufacturing the same.
2. Description of the Related Art
A vertical power MOSFET has an on-resistance that greatly depends on the electric resistance of a conduction layer (drift layer). The electric resistance of the drift layer is determined from the impurity concentration thereof. A higher impurity concentration results in a lower on-resistance. The higher impurity concentration though lowers the breakdown voltage across a PN junction formed between the drift layer and a base layer. Therefore, it is not possible to increase the impurity concentration above a limit determined on the basis of the breakdown voltage. In this way, there is a tradeoff between the breakdown voltage and the on-resistance of the element. An improvement in the tradeoff is an important subject to provide a semiconductor element of low power consumption. The tradeoff has a limit determined from material of the element. Overcoming the limit is a way to realize a semiconductor element of low on-resistance.
As an example of the MOSFET to solve this problem, a structure has been known, which is referred to as a superjunction structure that includes p-type pillars and n-type pillars buried laterally and alternately in a drift layer (see JP-A 2001-298181). In the superjunction structure, the amounts of charges (the amounts of impurities) contained in p-type pillars and n-type pillars are made equal to form a pseudo non-doped layer. This is effective to retain a high breakdown voltage and allow current to flow through highly doped n-type pillars, thereby realizing a lower on-resistance than the limit due to the material.
The superjunction structure is a technology to reduce a drift resistance of the drift layer, part of the on-resistance of the element. On the other hand, in a general structure, if the necessary maximum breakdown voltage of the element lowers, the proportion of the drift resistance in the on-resistance lowers. For example, the proportion of the drift resistance in the on-resistance is about 95% in a power MOSFET having a 600 V breakdown voltage. To the contrary, it lowers to 80% in an element having a 100 V breakdown voltage and 40% in an element having a 30 V breakdown voltage. As the necessary maximum breakdown voltage lowers, the proportion of the channel resistance increases while the proportion of the drift resistance in the on-resistance lowers. An increase in channel density inside the element is important to lower the channel resistance. For that purpose, it is effective to narrow the cell pitch between transistors. As for the gate electrode, the cell pitch can be made narrower in a trench gate structure, which includes a gate electrode buried in a trench formed in a base layer, than in a planar gate structure, which includes a planar gate electrode formed on the surface of a base layer with an insulator film interposed therebetween.
When the gate electrode having the trench gate structure is used, a reduction in channel resistance requires arrangement of the gate electrode on the n-type pillar in the superjunction structure without extending off the n-type pillar. A gate electrode extending to the p-type pillar narrows the current path and increases the channel resistance when the element is made conductive. The superjunction structure can be easily depleted on application of a high voltage if it has a reduced pillar pitch. Correspondingly, it is possible to increase the impurity concentrations in the pillars and reduce the on-resistance. Therefore, from the viewpoint of reducing the on-resistance and fine patterning the element, attempts have been made to reduce the pillar pitch in the superjunction structure. In consideration of the increase in channel resistance based on the occurrence of misalignment with the gate electrode as described above, fine patterning of the pillar pitch is limited.